The impact of representative paths of engine degradation on aircraft cruise NOx emissions was investigated. Engine cycles corresponding to older, current, and future subsonic and supersonic technologies were developed and used to determine the sensitivities of combustor conditions to various changes in component efficiencies and flow capacities due to aging. Estimates of relative changes in NOx emissions levels were made using established empirical correlations that relate emissions to combustor flow parameters. The analysis methodology was validated through comparisons to test data where available.
It was found that the sensitivity of specific fuel consumption (SFC) and combustor flow parameters to component aging is enhanced by increases in cycle temperatures and pressures. This ultimately results in a higher sensitivity of NOx emissions to engine degradation for cycles representative of more advanced technology. At constant thrust, turbine degradation typically acts to decrease NOx emissions while compressor aging results in an increase in NOx emissions; both occurring at the expense of SFC. Sample degradation scenarios were used to highlight the balance between turbine and compressor aging effects. Changes in NOx emissions in the −1% to +4% range were predicted for typical aging scenarios. The applicability of these results to the formulation of aircraft emissions inventories is considered.